If your main concern is fuel efficiency, the Maruti Suzuki Victoris is the better option.
The Kia Seltos delivers:
Petrol mileage between 17.0 - 17.9 km/l
Diesel mileage between 19.1 - 20.7 km/l
As per Maruti, the Maruti Victoris mileage figures are higher:
Petrol MT: 21.18 km/l
Petrol AT: 21.06 km/l
Petrol AllGrip AT: 19.07 km/l
CNG: 27.02 km/kg
Strong Hybrid (e-CVT): 28.65 km/l
Kia Seltos mileage is competitive in its diesel variants, but the Maruti Suzuki Victoris mileage clearly stands out, especially with the CNG and strong hybrid options. If mileage is your top priority, the Victoris is the better pick.

